Controlling P–C/C–H Bond Cleavage in Nickel Bis(diphosphine) Complexes: Reactivity Scope, Mechanism, and Computations

The synthesis of
heteroleptic [Ni­(P2N2)­(diphosphine)]­[BF4]2 complexes and the cleavage of P–C and
C–H bonds of the P2N2 ligand in those
complexes are reported. The products are five-coordinate complexes
in which Ni–C and P–H bonds have formed to give a cyclic
moiety containing Ni–CHNR2. The reactivity
of [Ni­(P2N2)­(diphosphine)]­[BF4]2 complexes is influenced by the rigidity of the diphosphine,
the steric effect of the substituents, and length of the carbon linker
of the diphosphine ligands. Diphosphine ligands bearing a rigid backbone
(e.g., dmpbz, 1,2-bis­(dimethylphosphino)­benzene) or aromatic substituents
(e.g., dppe, 1,2-bis­(diphenylphosphino)­ethane) react with [Ni­(PtBu2NBn2)­(CH3CN)2]­[BF4]2 to give P–C/C–H
bond cleavage products. Both [Ni­(PtBu2NBn2)­(dmpe)­(MeCN)]­[BF4]2 and [Ni­(PtBu2NBn2)­(dmpm)­(MeCN)]­[BF4]2 (dmpm
= 1,2-bis­(dimethylylphosphino)­methane) were prepared by the reaction
of [Ni­(PtBu2NBn2)­(CH3CN)2]­[BF4]2 with the corresponding diphosphine ligands. [Ni­(PtBu2NBn2)­(dmpe)­(MeCN)]­[BF4]2 readily undergoes P–C/C–H bond
cleavage in nitromethane. In sharp contrast, [Ni­(PtBu2NBn2)­(dmpm)]­[BF4]2 is stabilized by dmpm, a diphosphine with small bite
angle, and does not show P–C/C–H bond cleavage reactivity.
Computational results show that for complexes bearing less bulky diphosphine
ligands, such as dmpm, the barriers for the rate-determining transition
states are in some examples higher than 30 kcal/mol with the M06 functional,
higher than those for complexes bearing more rigid or more bulky ligands,
consistent with experimental studies. The calculated barriers for
the first transition state correlated with increased values of the
dihedral angle formed by the two NiP2 planes.